1. Field of the Invention
The present invention relates to an ink jet printing apparatus and method for controlling print head temperature, and particularly to a configuration for controlling the temperature of ink ejecting print heads, i.e. the temperature of ejected ink.
2. Description of the Related Art
Traditionally, control of the temperature of the ink in the print head has been carried out to control the variation of the volume of an ejected ink drop with respect to ink jet printing apparatuses. This can inhibit the occurrence of density variation in printed images. On the other hand, the temperature of the ink in the print head (also simply referred to herein as the temperature of the print head or the head temperature) changes in accordance with the ejection frequency of the ink and the ejection rest interval. For example, when an interruption of the printing operation (for example, a recovery operation or waiting for print data transmission) occurs during the printing operation, the temperature of the print head drops and a striking difference in the print density before and after the print interruption occurs.
As a configuration for the prevention of density variation in images caused by this kind of print head temperature variation, it is disclosed in Japanese Patent Application Laid-Open No. H08-039807 (1996) that the difference between a stable ejection temperature, which is set in advance and is the temperature at which print head ejection is stable, and the actual temperature of the print head is obtained, and that the print head temperature is controlled in response to this difference. More concretely, it is disclosed that when the difference is such that the stable ejection temperature is higher and is positive difference, heating is carried out, and conversely the non-print operation time is extended when there is a large negative difference.
It is also disclosed in Japanese Patent Application Laid-Open No. H04-193537 (1992) that when restarting a printing operation after an interruption the head temperature is controlled such that it becomes roughly the same temperature as at the time of interrupting the printing operation.
However, problems such as those described below often arise in the prior art temperature controls described in the above two publications. As for Japanese Patent Application Laid-Open No. H04-193537 (1992), when a scan of the print head is completed and the printing operation is interrupted, the head temperature at the time of completion becomes the target temperature of the temperature control at the time of the next printing. Therefore, for example, when the head temperature rises due to a scan in which high density images are printed, that is, images with a high ejection frequency, control is carried out with the raised temperature at the time of scan completion as the target and as such the head temperature is prone to reach a higher temperature.
FIG. 12A is a diagram that, at times when there are printing interruptions due to, for example, print data transmission waiting time, illustrates head temperature variation versus the passage of time, during the printing of an image with a comparatively high density, in connection with the temperature control described in Japanese Patent Application Laid-Open No. H04-193537 (1992). In the case where a high density image is printed, the ejection frequency of the print head has thereby increased and thus the temperature rise of the print head becomes larger. Because of this, in the example shown in the same figure, the head temperature rises to approximately 50° C. in the first three times of printing operations (scans). Subsequently, when there is a print interruption due to, for example, print data transmission waiting time, the head temperature has a relatively large decrease due to the cessation of the ejection operation. Thus, in the subsequent head temperature control, heating is carried out with 50° C., the head temperature at the time of interrupting printing operation (the time at the completion of the immediately preceding scan), as the target temperature, and printing is restarted. In this printing as well, when the print density is high, the head temperature rises in the restarted scan as well. In this manner, in the case where the density of the printed image is high, the above described temperature change repeats and as a result the head temperature increasingly elevates.
Also, as for Japanese Patent Application Laid-Open No. H08-039807 (1996), in the case where the head temperature at the time of scan completion is lower than the predetermined stable ejection temperature, heating occurs before the next scan until the stable ejection temperature is reached. Therefore, in the case where density of an image to be printed is low and thus the head temperature is lowered a comparatively large amount during scan, the difference between the lowered head temperature at the time when the scan is completed and the head temperature at the start of the next scan, which is obtained by heating the print head to the stable ejection temperature, becomes large, and a large density difference occurs between the images of the scans.
FIG. 12B is a diagram that illustrates the head temperature variation when a low density image is printed while the head temperature control described in Japanese Patent Application Laid-Open No. H08-039807 (1996) is carried out. In this example the stable ejection temperature is 40° C. The head temperature, which has risen to the stable ejection temperature of 40° C., decreases due to the printing of a low density image, that is, the printing of an image with a low ejection frequency. In the example shown in FIG. 12B, in one scan it drops 8° C. from approximately 40° C. to 32° C. After the first scan has been completed, the head temperature is again heated to 40° C., which is the stable ejection temperature, before the next scan. As a result the difference between the head temperature at the time the scan is completed (32° C.) and the head temperature at the time that the next print scan is commenced, that is, the stable ejection temperature (40° C.), becomes a relatively large 8° C. Accordingly, density variation occurs as the result of the head temperature difference at each scan.
On the other hand, in the head temperature control of Japanese Patent Application Laid-Open No. H08-039807 (1996), when the head temperature at the time of scan completion is higher than the predetermined stable ejection temperature, density variation also occurs in the same manner. That is, in the case where the head temperature at the time a scan has been completed is higher than the stable ejection temperature, the non-print operation time is extended in order to decrease the head temperature. Therefore, also in the case where the print density is high the difference between the head temperature at the time of scan start and the head temperature at the time of scan completion is large, and density variation in the image is prone to occur.
FIG. 12C is a diagram that illustrates head temperature variation in Japanese Patent Application Laid-Open No. H08-039807 (1996) when an image with a high print density is printed. The ejection frequency increases due to the printing of an image with a high print density, and such scan raises the head temperature. In the example shown in FIG. 12C head temperature rises 8° C. in one scan. Because the head temperature at the time that a scan is completed (48° C.) is higher than the stable ejection temperature, the non-print operation time after the scan is extended and printing is paused until the head temperature drops to 40° C. Accordingly, the difference between the head temperature at the time of scan completion (48° C.) and the head temperature at the time of start of the next scan (40° C.) becomes relatively large, and density variation becomes prone to occur due to the head temperature difference.
As explained above, the prior art head temperature controls, so to speak, are such that a predetermined temperature, i.e. the stable ejection temperature, or the head temperature at the time of scan completion, are made the target temperature of the temperature control of the next printing. As a result, in these head temperature controls there is a problem wherein head temperature variation is prone to become large.